Julius Caesar Gallic War 7.65.4

The only guards provided against all these contingencies were twenty-two cohorts, which were collected from the entire province by Lucius Caesar, the lieutenant, and opposed to the enemy in every quarter. The Helvii, voluntarily engaging in battle with their neighbors, are defeated, and Caius Valerius Donotaurus, the son of Caburus, the principal man of the state, and several others, being slain, they are forced to retire within their towns and fortifications. The Allobroges, placing guards along the course of the Rhine , defend their frontiers with great vigilance and energy. Caesar, as he perceived that the enemy were superior in cavalry, and he himself could receive no aid from the Province or Italy, while all communication was cut off, sends across the Rhine into Germany to those states which he had subdued in the preceding campaigns, and summons from them cavalry and the light-armed infantry, who were accustomed to engage among them. On their arrival, as they were mounted on unserviceable horses, he takes horses from the military tribunes and the rest, nay, even from the Roman knights and veterans, and distributes them among the Germans.